In general, a radio wave transmitted from a broadcasting satellite and received by a parabolic antenna, for example, a signal in frequency of 11.7.about.12.2 GHz is once group-converted into the first intermediate frequency signal of 950.about.1450 MHz by the low noise convertor LNC provided in the premises just under antenna and then supplied to an indoor unit through coaxial cable. An indoor unit selects the frequency of specific station from the first intermediate frequency signal supplied from LNC, then amaplifies and demodulates the VIDEO and AUDIO signals.
In such a receiving system of indoor unit, the final intermediate frequency is usually set to a frequency considerably lower than the receiving frequency, for example, to a frequency as low as 70 MHz. However, in this case, following problems remain unsolved.
1. In case the intermediate frequency to be input to the demodulator is only about 70 MHz in the single or double superheterodyne system, the specific bandwidth becomes broad in the PLL demodulation system, a simple VCO using only one variable capacitance diode cannot provide sufficient linearity and therefore an more expensive VCO assuring advanced linearity must be used.
2. When a demodulating system different from the PLL demodulating system is employed in above item 1, the intermediate frquency signal is partly used for AFC. In this case, since the C/N(carrier/random noise ratio) threshold extension effect is not applied on the signal, if C/N is bad, the AFC may operate errorneously.
3. In case the intermediate frequency is selected to 70 MHz, for example, in the single superheterodyne system, an image frqeuency is within the bandwidth of input signal, it is required to use the image rejection type mixer or to insert an RF tuning circuit which operates in conjunction with a channel selector before the mixer.